Adding missing override keywords

Prevents compiler warnings such as this:

```
/Users/Will/CLionProjects/OpenMW/apps/openmw/mwgui/windowbase.hpp:65:22: warning: 'onOpen' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
        virtual void onOpen();
                     ^
/Users/Will/CLionProjects/OpenMW/apps/openmw/mwgui/windowbase.hpp:38:22: note: overridden virtual function is here
        virtual void onOpen() {}
                     ^
```

apps/openmw/mwgui/messagebox.hpp

@@ -81,7 +81,7 @@ namespace MWGui
 
             MyGUI::Widget* getDefaultKeyFocus() override;
 
-            virtual bool exit() { return false; }
+            virtual bool exit() override { return false; }
 
             bool mMarkedToDelete;
 

apps/openmw/mwgui/windowbase.hpp

@@ -56,15 +56,15 @@ namespace MWGui
 
 
     /*
-     * "Modal" windows cause the rest of the interface to be unaccessible while they are visible
+     * "Modal" windows cause the rest of the interface to be inaccessible while they are visible
      */
     class WindowModal : public WindowBase
     {
     public:
         WindowModal(const std::string& parLayout);
-        virtual void onOpen();
-        virtual void onClose();
-        virtual bool exit() {return true;}
+        virtual void onOpen() override;
+        virtual void onClose() override;
+        virtual bool exit() override {return true;}
     };
 
     /// A window that cannot be the target of a drag&drop action.

apps/openmw/mwworld/worldimp.hpp

@@ -608,7 +608,7 @@ namespace MWWorld
             void launchProjectile (MWWorld::Ptr actor, MWWorld::ConstPtr projectile,
                                            const osg::Vec3f& worldPos, const osg::Quat& orient, MWWorld::Ptr bow, float speed, float attackStrength) override;
 
-            void applyLoopingParticles(const MWWorld::Ptr& ptr);
+            void applyLoopingParticles(const MWWorld::Ptr& ptr) override;
 
             const std::vector<std::string>& getContentFiles() const override;
             void breakInvisibility (const MWWorld::Ptr& actor) override;